CVE-2022-50488

In the Linux kernel, the following vulnerability has been resolved:

block, bfq: fix possible uaf for 'bfqq->bic'

Our test report a uaf for 'bfqq->bic' in 5.10:

================================================================== BUG: KASAN: use-after-free in bfq_select_queue+0x378/0xa30

CPU: 6 PID: 2318352 Comm: fsstress Kdump: loaded Not tainted 5.10.0-60.18.0.50.h602.kasan.eulerosv2r11.x86_64 #1 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58-20220320_160524-szxrtosci10000 04/01/2014 Call Trace: bfq_select_queue+0x378/0xa30 bfq_dispatch_request+0xe8/0x130 blk_mq_do_dispatch_sched+0x62/0xb0 __blk_mq_sched_dispatch_requests+0x215/0x2a0 blk_mq_sched_dispatch_requests+0x8f/0xd0 __blk_mq_run_hw_queue+0x98/0x180 __blk_mq_delay_run_hw_queue+0x22b/0x240 blk_mq_run_hw_queue+0xe3/0x190 blk_mq_sched_insert_requests+0x107/0x200 blk_mq_flush_plug_list+0x26e/0x3c0 blk_finish_plug+0x63/0x90 __iomap_dio_rw+0x7b5/0x910 iomap_dio_rw+0x36/0x80 ext4_dio_read_iter+0x146/0x190 [ext4] ext4_file_read_iter+0x1e2/0x230 [ext4] new_sync_read+0x29f/0x400 vfs_read+0x24e/0x2d0 ksys_read+0xd5/0x1b0 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x61/0xc6

Commit 3bc5e683c67d ("bfq: Split shared queues on move between cgroups") changes that move process to a new cgroup will allocate a new bfqq to use, however, the old bfqq and new bfqq can point to the same bic:

1. Initial state, two process with io in the same cgroup.

Process 1 Process 2 (BIC1) (BIC2) | Λ | Λ | | | | V | V | bfqq1 bfqq2

2. bfqq1 is merged to bfqq2.

Process 1 Process 2 (BIC1) (BIC2) | | -------------| V bfqq1 bfqq2(coop)

3. Process 1 exit, then issue new io(denoce IOA) from Process 2.

(BIC2) | Λ | | V | bfqq2(coop)

4. Before IOA is completed, move Process 2 to another cgroup and issue io.

Process 2 (BIC2) Λ |--------------
| V bfqq2 bfqq3

Now that BIC2 points to bfqq3, while bfqq2 and bfqq3 both point to BIC2. If all the requests are completed, and Process 2 exit, BIC2 will be freed while there is no guarantee that bfqq2 will be freed before BIC2.

Fix the problem by clearing bfqq->bic while bfqq is detached from bic.

On Red Hat Enterprise Linux 9, regular (non-root) users cannot exploit this vulnerability. This is due to the following:

1. By default, SELinux actively enforces policies in targeted mode, meaning that even if base permissions allow cgroup manipulation, unprivileged cgroup manipulation will be prevented.
2. By default, systemd does not delegate cgroup control to user processes, meaning that user sessions and services cannot create or manage their own cgroup hierarchies.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.